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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
</h3></div></div></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
        models</a>
</h4></div></div></div>
<p>
          In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
          <code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
          variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
          (cf. 8.5)). When a declaration includes a non-empty initializer (an initial
          value is given), it is said that the object has been initialized. If the
          declaration uses an empty initializer (no initial value is given), and
          neither default nor value initialization applies, it is said that the object
          is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
          but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
          <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          intends to formalize the notion of initialization (or lack of it) allowing
          a program to test whether an object has been initialized and stating that
          access to the value of an uninitialized object is undefined behavior. That
          is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> and no initial value is given, the
          variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
          optional object has conceptually no value at all and this situation can
          be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
          to try to access the value of an uninitialized optional. An uninitialized
          optional can be assigned a value, in which case its initialization state
          changes to initialized. Furthermore, given the formal treatment of initialization
          states in optional objects, it is even possible to reset an optional to
          <span class="emphasis"><em>uninitialized</em></span>.
        </p>
<p>
          In C++ there is no formal notion of uninitialized objects, which means
          that objects always have an initial value even if indeterminate. As discussed
          on the previous section, this has a drawback because you need additional
          information to tell if an object has been effectively initialized. One
          of the typical ways in which this has been historically dealt with is via
          a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
          <code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
          equivalent to adding the special value to the set of possible values of
          a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
          plus some <span class="emphasis"><em>nil_t</em></span>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
          is some stateless POD—can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
          unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
          <span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
          Using the <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
          library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>. There is precedent for a discriminated
          union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
          <span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
          a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
          serves as a conceptual foundation.
        </p>
<p>
          A <code class="computeroutput"><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code> follows naturally from the traditional
          idiom of extending the range of possible values adding an additional sentinel
          value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
          this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
          for our purpose since our goal is to formalize the notion of uninitialized
          objects and, while a special extended value can be used to convey that
          meaning, it is not strictly necessary in order to do so.
        </p>
<p>
          The observation made in the last paragraph about the irrelevant nature
          of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
          with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
          suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
          has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
        </p>
<p>
          As of this writing I don't know of any precedent for a variable-size fixed-capacity
          (of 1) stack-based container model for optional values, yet I believe this
          is the consequence of the lack of practical implementations of such a container
          rather than an inherent shortcoming of the container model.
        </p>
<p>
          In any event, both the discriminated-union or the single-element container
          models serve as a conceptual ground for a class representing optional—i.e.
          possibly uninitialized—objects. For instance, these models show the
          <span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
          values:
        </p>
<p>
          Discriminated-union:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
              variant implies copies of the value.
            </li>
<li class="listitem">
              <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
              between variants matches both current types and values
            </li>
<li class="listitem">
              If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
              it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
            </li>
<li class="listitem">
              If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
              it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
            </li>
<li class="listitem">
              Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
              models testing if the optional is initialized
            </li>
<li class="listitem">
              Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
              from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
              models the undefined behavior of trying to access the value of an uninitialized
              optional
            </li>
</ul></div>
<p>
          Single-element container:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
              container implies copies of the value.
            </li>
<li class="listitem">
              <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
              between containers compare container size and if match, contained value
            </li>
<li class="listitem">
              If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
              optional.
            </li>
<li class="listitem">
              If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
              optional.
            </li>
<li class="listitem">
              Testing if the container is empty models testing if the optional is
              initialized
            </li>
<li class="listitem">
              Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
              from an empty container models the undefined behavior of trying to
              access the value of an uninitialized optional
            </li>
</ul></div>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 Andrzej Krzemieński<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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